Due to their extraordinarily good ratio of stiffness or strength to density, core composites, i.e. composite sandwich structures having lightweight cores, have a broad range of application especially in the field of aircraft construction.
Well-known conventional core composites are generally formed of an upper and a lower cover layer or cover ply, between which is located, for example, a honeycomb-type core structure formed of vertically extending cells with a hexagonal cross section, for increasing the stiffness of the resulting composite sandwich structure.
For example metallic corrosion-protected aluminum foils, or non-metallic materials, such as Nomex®- or Kevlar®/N636-paper for example, are used for forming the core structure. Both the Nomex®-paper as well as the Kevlar®/N636-paper are coated with phenolic resin in a submersion process for increasing the mechanical strength thereof. Finally, the core structure is provided on both sides with cover layers, to form the finished or complete core composite.
A finished core having the above described true honeycomb structure can be readily provided with a local reinforcement area without problems. Due to the small-volume repeat units represented by the individual hexagonal honeycomb cells extending perpendicular to the cover layers, a stiffening or reinforcing material for forming a reinforcement area can be filled, pressed, foamed, poured or cast directly into a bored hole that passes through the core composite, and remains spatially confined within the cell or cells that is or are opened by the bored hole. The stiffening material may, for example, be a curable synthetic plastic material, for example a synthetic resin and/or synthetic plastic foam. In order to enable a connection of the core composite to other components, the core composite can then be bored in the reinforcement area. If necessary, by means of pertinent known methods, a securing element, for example an insert nut or some other insert, can also be introduced into the reinforcement area of the core composite.
In contrast to the above described true honeycomb-type core composites, in which a spatial limitation is always provided by the relatively small, individual cells extending perpendicular to the cover layers, new types of core composites, especially formed of three-dimensional folded comb or cell structures, comprise an open or drainable structure. Namely, such pleated or folded core structures include fold or pleat valleys that form open channels extending continuously in the plane of the composite structure, i.e. along or parallel to the cover layers from edge-to-edge of the composite structure. Thus, the core channels of such a core composite extend open and unobstructed through large areas (or even along the entire length or width) of the finished composite sandwich. Thereby, for example, it is possible to guide various types of lines (e.g. electrical lines, hydraulic lines, water lines, air lines, etc.) through the core composite without previously having to cut or machine a passage therethrough while impairing the mechanical properties of the core composite.
If, in order to form reinforcement areas in the above described complete core composites with drainable core structures, a pasty or viscous hardening material is introduced, for example, into a bored hole passing through the core composite, the material can thus, depending on its viscosity, more or less uncontrolledly spread out laterally through the open channel or channels of the core structure. Namely, viscous or pasty substances can spread-out uncontrollably over larger distances in open drainable core structures, in contrast to true honeycomb structures, in which fundamentally only a limited number of structural units is opened by a bored hole and thus introduced substances are always spatially limited.
Therefore the standard reinforcement methods according to the state of the art are not usable without further measures for producing reinforcement areas within complete core composites having an open drainable core structure between two cover layers.